1. Field of the Invention
The present invention relates to an electronic dimming ballast, and more particularly, to a method of determining an approximation of a resonant frequency of a resonant tank circuit of an electronic dimming ballast, and adjusting an operating frequency of the ballast in response to the approximation of the resonant frequency.
2. Description of the Related Art
Prior art electronic ballasts for fluorescent lamps typically comprise a “front-end” circuit and a “back-end” circuit. The front-end circuit often includes a rectifier for receiving an alternating-current (AC) mains line voltage and producing a rectified voltage VRECT, and a boost converter for receiving the rectified voltage VRECT and generating a direct-current (DC) bus voltage VBUS across a bus capacitor. The boost converter is an active circuit for boosting the magnitude of the DC bus voltage above the peak of the line voltage and for improving the total harmonic distortion (THD) and the power factor of the input current to the ballast. The back-end circuit typically includes a switching inverter circuit for converting the DC bus voltage VBUS to a high-frequency inverter output voltage VINV (e.g., a square-wave voltage), and a resonant tank circuit for generating a sinusoidal voltage VSIN from the inverter output voltage VINV and coupling the sinusoidal voltage VSIN to the lamp electrodes. The amount of power delivered to the lamp may be adjusted by controlling a duty cycle DCINV of the inverter output voltage VINV to thus control the intensity of the lamp from a low-end intensity LLE to a high-end intensity LHE. An operating frequency fOP of the inverter output voltage VINV may be held constant for much of the dimming range of the lamp between the low-end intensity LLE to the high-end intensity LHE.
In order for the resonant tank circuit to provide an appropriate amount of output impedance to the lamp, such that the lamp intensity is stable and does not flicker when controlled to the low-end intensity LLE, the operating frequency fOP of the inverter output voltage VINV is typically controlled to a low-end frequency fLE that is slightly above a resonant frequency fRES of the resonant tank circuit at the low-end intensity LLE. However, if the operating frequency fOP of the inverter output voltage VINV is controlled too close to the resonant frequency fRES, the reverse recovery of diodes in the inverter circuit may cause noise and increased temperatures in the inverter circuit. Therefore, there is a frequency window above the resonant frequency fRES in which the operating frequency fOP of the inverter output voltage VINV must be controlled when the lamp is at the low-end intensity LLE. Since the resonant frequency fRES is dependent upon the tolerances of the components of the resonant tank circuit, the components of the resonant tank circuit as well as the value of the low-end frequency fLE must be carefully chosen to ensure that the operating frequency fOP of the inverter output voltage VINV is within the frequency window when the lamp is at the low-end intensity LLE. Accordingly, there is a need for an electronic dimming ballast that is able to more accurately control the operating frequency fOP of the inverter output voltage VINV with respect to the resonant frequency fRES when the lamp intensity is controlled near the low-end intensity LLE.